Release captive-bred reptiles into the wild: Snakes & lizards

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Study locations

Key messages



  • Abundance (2 studies): One global review found that when using recruitment to the adult population as a measure of success, 32% of reptile releases (releases of captive individuals were 7% of total projects) were successful. One review in New Zealand found that 13% of lizard releases (some involving captive-bred animals) found evidence of populations growth
  • Reproductive success (3 studies): Three studies (including two reviews) in the USA and New Zealand found evidence of breeding following release in one of two captive-bred populations of cornsnakes, one captive-bred population of Otago skinks and in at least 16 lizard mitigation translocations, some of which involved captive-bred animals.
  • Survival (9 studies): One replicated, controlled study in Spain found that released large psammodromus lizards had similar annual survival compared to resident lizards. Two of six studies (including one replicated study and two reviews) in the USA, Australia, New Zealand and Canada reported that 13% of 40 indigo snakes were re-sighted at least once during 5–8 years following release or that 58% of 12 Otago skinks survived at least 18 months. Two studies found that zero of nine and 27 individuals survived more than 143 days or beyond their first hibernation. The other two studies found that one of two and five of 53 releases (only some of which involved captive-bred animals) failed completely (no individuals survived). One study in New Zealand found that survival of captive-bred Otago skinks released into an enclosure was higher when mice had been eradicated compared to when skinks were released in the presence of mice. One replicated study in the Galápagos found that while releases were ongoing over a decade (183 released in total), 17–32 Galápagos land iguanas were recaptured each year.
  • Condition (1 study): One controlled study in New Zealand found that body condition of captive-reared Otago skinks was higher than wild skinks, but sprint speed was lower.


  • Behaviour change (1 study): One replicated, controlled study in Spain found that released large psammodromus lizards moved between habitat fragments more frequently than resident lizards but showed similar behaviour in three other measures.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A review in 1991 of reptile translocation and release programmes in New Jersey, USA (Reinert 1991) found that of two releases of captive-bred, newly born cornsnake Elaphe guttata, one population survived at least four years, while the other survived one–two years. In one site, 17 of 158 (11%) released newly born captive-bred cornsnakes survived at least one year and six released snakes survived at least 4 years and bred. In a second site, six of 262 (2%) released snakes were recaptured one year after release and none were recaptured two years after release. In the first site, 158 newly born captive-bred cornsnakes were released into a known hibernaculum in 1982–1988. In the second site, 262 newly born captive-breds were released in 1985–1989.

    Study and other actions tested
  2. A study in 1980–2001 on an island off the coast of Florida, USA (Irwin et al. 2003) found that a small number of released (some captive-bred) eastern indigo snakes Drymarchon couperi survived 5–8 years in the wild. In the 17–20 years after 40 eastern indigo snakes were released, five snakes were recorded in the wild and the last snake was observed 5–8 years after release (1983: 1 individual; 1985: 1 individual; 1986: 2 individuals; 1988: 1 individual). In 1980–1982, forty eastern indigo snakes (hatchlings and juveniles from a captive breeding colony, wild-caught adults, confiscated snakes and donated from zoos) were released onto St Vincent Island National Wildlife Refuge (51 km2). Snakes were monitored using combinations of cameras in gopher tortoise Gopherus polyphemus burrows and drift fence/pitfall trap arrays in autumn, winter and spring 1983–1990, January and December 2000, and April 2001. Sightings (unverified) were also recorded but are not reported here.

    Study and other actions tested
  3. A replicated study in 1991–1993 on a tropical island in the Galápagos, Ecuador (Phillips et al. 2005) found that following release of captive-bred Galápagos land iguanas Conolophus subcristatus, some survived and reproduced. Between 17–32 iguanas were recaptured/year. More offspring of released iguanas were captured after most cats Felis catus were eradicated from the island (1 and 14 adults and 6 and 14 sub-adults and juveniles/year) than before the cat control program began (1 and 0 adults and 6 and 4 sub-adults and juveniles/year). In 1991–2003, a total of 183 captive-bred iguanas were released over six releases (15–63 released every 1–5 years). Cat eradication started in 2001 and was completed in 2003. Iguanas were surveyed (6 days in June–July) before (1999–2000) and after (2002–2003) the majority of cat eradication had been completed.

    Study and other actions tested
  4. A review of worldwide reptile translocation projects during 1991–2006 (Germano & Bishop 2009) found that a third of the projects, that included some releases of captive-bred animals, were considered successful with substantial recruitment to the adult population. Of the 47 translocation projects reviewed (39 species), 32% were successful, 28% failed and long-term success was uncertain for the remaining 40%. Projects that translocated animals due to human-wildlife conflicts failed more often (63% of 8 projects) than those for conservation purposes (15% of 38) and those for research purposes (50% of 5). Success was independent of the life-stage translocated/released, number of animals released and geographic region (see paper for details). Releases of captive-bred animals made up 7% of the projects, and individuals involved were adults in 75% of cases, juveniles and sub-adults in 64% of cases and eggs in 4% of cases. The most common reported cause of failure was homing and migration with the second most common reported cause being insufficient numbers, human collection and food/nutrient limitation all equally reported. Success was defined as evidence of substantial recruitment to the adult population during monitoring over a period at least as long as it takes the species to reach maturity.

    Study and other actions tested
  5. A replicated, controlled study in 2001–2006 in two sites of forest fragments among cereal field in northern Spain (Santos et al. 2009) found that released captive-bred large psammodromus lizards Psammodromus algirus had similar survival compared to resident lizards, and a newly established population persisted for at least four years. Survival for a year after release was similar for released captive-bred lizards (2001–2002: 26 of 178, 15%; 2002–2003: 19 of 187, 10%) and residents (2002–2003: 4 of 30, 13%). A release site where only three native lizards were capture in 2002 still hosted a population in in 2006 (at least 6 individuals). More captive-bred lizards moved between habitat fragments (8 of 48 lizards) than did residents (2 of 112 lizards), though three other measures of movement and activity were similar between captive-bred and resident lizards (see paper for details). Captive-bred lizards were released in groups of 5–7 in two woodland fragments (0.9–5.2 ha) in each of two sites (located 8 km apart, two fragments/site). One site had a viable resident lizard population while the other did not. Released and resident lizards were monitored in spring and summer by walking around study sites and adjacent areas and noosing all detected lizards in 2002, 2003 (15 days each) and 2006 (two days). 

    Study and other actions tested
  6. A study in 2007 in a site of mixed sand dunes, Acacia spp. and shrubland in South Australia (Read et al. 2011) found that releasing captive-bred woma pythons Aspidites ramsayi into a large, fenced enclosure was unsuccessful due to predation. All pythons died between 41 and 123 days after release, all most likely due to attack or predation by mulga snakes Pseudechis australis. Two snakes had lost weight (10–37% of release mass) but were not considered emaciated. Nine captive-bred sibling pythons (two females, seven males, hatched in 2002) had radio transmitters surgically implanted in April 2007. They were released in September 2007 (weighing 890–1,350 g) into a large enclosure (60 km2) free of non-native mammalian predators. The snakes were from a wild stock originating from close to the release site. Four snakes were released into a release-pen, but all escaped within two months. Pythons were located daily until death.

    Study and other actions tested
  7. A controlled study in 2009–2011 in one site of temperate shrubland in Alexandra, New Zealand (Hare et al. 2012) found that releasing captive-reared Otago skinks Oligosoma otagense into a fenced enclosure resulted in some surviving for at least 18 months and some breeding successfully. Most captive-bred skinks survived for at least 12–18 months after release (12 months: 75%, 9 of 12 skinks survived, 3 females disappeared; 18 months: 58% survival, 2 males disappeared). Three newborn young were recorded two years after release.  Body condition of captive-reared skinks was higher than wild skinks (various species of Oligosoma skinks; presented as condition index), but sprint speed was lower (captive-bred: 0.4–0.6 m/s; wild: 0.9–1.5 m/s). In November 2009, twelve skinks from captive stock (five males, seven females, at least 3rd generation captive-born) were released. The release site (0.3 ha) was surrounded by a 1.9 m high mammal-proof fence and was free of all mammals for five months prior to release. Post-release visual monitoring using a camera to photograph and identify all lizards seen at sunning spots (rocks) was performed for two hours, 1–5 times/month from November 2009 to May 2011 (43 searches). Sprint speed was measured for skinks in captivity (29 skinks) and those in the wild (93 skinks).

    Study and other actions tested
  8. A replicated study 2006 in a nature reserve within a wider urban setting in Ontario, Canada (Harvey et al. 2014) found that captive born massasauga rattlesnakes Sistrurus catenatus catenatus released into the wild did not survive hibernation. Following release, at least 19 of 27 (70%) of rattlesnakes survived 19 weeks to hibernation (three died from predation, three transmitters failed, one died from human attack, one died from unknown causes). No rattlesnakes survived the hibernation period (10 died from exposure on the surface, four died from predation, four died from unknown causes, one was killed by human attack). In 2003, two gravid female rattlesnakes were rescued from a development site in Windsor, Ontario, Canada and their young raised in captivity. In 2006, the 3-year-old snakes (27 individuals) were implanted with radio transmitters and released into a nature reserve which had a natural population of rattlesnakes until at least the mid-1970s. Snakes were tracked daily for the first two weeks after released and then fortnightly thereafter.

    Study and other actions tested
  9. A study in 2009–2012 in an area of mixed shrub and grassland in Otago, New Zealand (Norbury et al. 2014) found that survival of captive-bred Otago skinks Oligosoma otagense released into an enclosure was higher for those released when house mice Mus musculus had been eradicated compared to when skinks were released in the presence of mice. Authors reported that post-release survival was higher for skinks released with no mice present (44%) compared to survival of skinks released just prior to reinvasion by mice (15%; see paper for details). Survival of established skinks (2 years after their release) after the mouse reinvasion was higher (91%) than for newly released skinks in the presence of mice (17%). In 2009, a 0.3 ha area was enclosed within a mammal resistant fence (1.9 m high), and 12 captive-bred adult skinks were released in the enclosure following eight weeks in quarantine. In 2011, an additional 16 skinks were quarantined and released. Over a six-month period prior to the release, all mammals inside the enclosure were eradicated using a range of baited traps. House mice reinvaded during 2012 and were again eradicated using live capture traps and poison bait stations. In 2009–2012, starting 7–10 days after release, skinks were monitored every 15 days by a walking survey of the enclosure.

    Study and other actions tested
  10. A review published in 2016 of lizard translocation and release projects (some involving captive-bred animals) in New Zealand during 1988–2013 (Romijn & Hartley 2016) found that most projects found evidence of breeding following release, but few found evidence of population growth. Forty-five of 53 (85%) translocations/releases motivated by conservation had some post-release monitoring. Seven found evidence of population growth (more lizards found than released), 33 found that populations were smaller than the number released, at least 16 found evidence of breeding after release, and five resulted in complete failure (no lizards found). One translocation (of Oligosoma infrapunctatum) was later discovered to be to a location outside the species historic range. Some translocations/releases involved wild animals and others captive bred animals (project success vs source of animals not stated). Published and unpublished literature were searched, and key people associated with each project were identified and contacted for further information. Translocations/releases were considered to be motivated by conservation if the primary focus was to benefit the species or recipient site.

    Study and other actions tested
Please cite as:

Sainsbury K.A., Morgan W.H., Watson M., Rotem G., Bouskila A., Smith R.K. & Sutherland W.J. (2021) Reptile Conservation: Global Evidence for the Effects of Interventions for reptiles. Conservation Evidence Series Synopsis. University of Cambridge, Cambridge, UK.

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